Fibrous materials are used for numerous industrial purposes, such as for making insulation products, structural products, reinforcement products, and decorative products. Insulation products include thermal insulation products, which block heat flow, and acoustical insulation products, which can have either a sound-barrier quality to block the transmission of sound or a sound-absorptive quality to dissipate or absorb sound, or both a sound-barrier quality and a sound-absorptive quality. Fibrous thermal and acoustical insulation products are commonly made from relatively short fibers in the form of a wool. For example, building insulation is typically fibrous glass wool. Glass wool fibers have an average length significantly less than about 1 cm, and an average diameter of about 4 to about 20 microns. Glass wool is typically made in a rotary process by centrifuging molten glass from a rotating centrifuge or spinner. The rotary process has great variability, resulting in wide ranges in the length and diameter of glass fibers. Besides glass fibers, other inorganic and organic fibers can be made using the rotary process.
Continuous inorganic fibers are typically made from stationary multi-holed bushings, whereas continuous organic fibers are usually made from stationary spinnerets. In either case, fibers are made by mechanically pulling or attenuating the fibers from molten material. Continuous fiber-forming processes are operated under closely controlled conditions, and the fibers have generally constant, uniform diameters. Continuous fibers are formed individually, and are gathered and collected as a multifilament strand of continuous filaments. The fibers are long, and can be considered to be unending (i.e., the fibers have an essentially infinite length) unless broken. Generally, continuous fibers have strengths several orders of magnitude greater than those of rotary fibers, but the manufacturing cost for rotary fibers is significantly less than the cost for continuous fibers. An important advantage of continuous fibers is that the continuous fiber process can operate at higher temperatures. Therefore, the fibrous materials making up the continuous fibers can have higher softening points and can operate at higher service temperatures than possible with the rotary wool materials. For example, E-glass fibers can be made using the continuous fiber process, but not with a rotary process.
One of the uses for the discrete-length inorganic wool fibers produced by the rotary process is in molded wool products. For example, glass wool having about 20 percent by weight organic binder is molded into pipe insulation for its thermal insulation value. Glass wool is also molded into acoustical insulation products, such as appliance insulation and automobile headliners. Further, glass wool can be molded into products having an important structural utility, such as glass wool window lineals.
One of the characteristics of molded rotary glass wool products is that high amounts of binder are required for desired product qualities. From a cost and an environmental standpoint it would be advantageous to be able to reduce the amount of binder in molded products. Another characteristic of molded rotary wool products is that since the glass wool is made of short fibers, during product use some of the fibers can become separated and become airborne. It would be beneficial if the rotary fibers could be bound up or prevented from being dislodged, especially since some of the dislodged fibers could be small enough in diameter to become inhaled during human respiration. The problem of dislodged or separated fibers is particularly troublesome where the molded wool product is subjected to mechanical or pneumatic pulsing or shock, such as a severe vibration during the service of the product.
Under conventional molding processes for products where the dislodging or separating of discrete fibers from the molded product is a concern, the edges of the molded product have been typically molded with a greater density than the average density of the product. The use of a greater density at the edges helps bind the product to prevent dislodging of the fibers. Some fibrous wool molding processes provide for densities in the edges of the product approaching or exceeding 200 percent of the average density of the product, which can necessitate the use of large amounts of wool in the edges.
In an attempt to overcome some of the drawbacks of rotary wool products, a wool-like material has been developed using continuous fibers. This material, referred to as continuous-filament wool, has the advantage of incorporating fibers so long that they can be considered to be essentially unending. The long nature of the fibers substantially decreases the amount of free fibers that can become dislodged from the product, even under extreme service conditions. Also, the continuous fibers have a controlled diameter, which can be set to be above the human-respirable range.
Acoustical insulation for filling automobile muffler shells is a known example of continuous-filament wool made from continuous fibers. Such a wool product is described in U.S. Pat. No. 4,569,471 to Ingemansson et al., the disclosure of which is hereby incorporated by reference. This patent discloses a process for filling muffler shells by advancing a multifilament strand of continuous filaments through a nozzle to separate the filaments from each other to form a continuous-filament wool. The glass wool in the muffler shell absorbs some of the sound energy, thereby appreciably reducing the noise of the engine. The continuous-filament wool is blown by the nozzle directly into the muffler shell, which is perforated to allow separation and evacuation of the air traveling with the fibers. Another advantage of the continuous-filament wool process is that the filaments can be made of a material having a higher softening point than possible with rotary-produced fibers. A problem of the continuous-filament wool collection process is that the density is limited to that which is possible by intercepting air-borne fibers in a perforated container.
An improvement in the use of continuous-filament wool as an acoustical absorber in automobile mufflers is disclosed in European Patent Publication No. EP 0692616 A1, published Jan. 17, 1996, the disclosure of which is hereby incorporated by reference. The continuous-filament wool process can be used to make an acoustical insulation in a separate perforated collection device rather than in the muffler. A binder material can be added as the filaments are drawn through a nozzle, and the wool with the binder can be heated in the perforated collection device to set or cure the binder, thereby making a rigid acoustical insulation product suitable for insertion in a muffler. This process has the disadvantage, however, that the density cannot be any greater than that which is possible by intercepting air-borne fibers in a perforated container.